1. Field of the Invention
The present invention relates to a method utilized in a wireless communication system, and more particularly, to a method of blind decoding of a control channel.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
A long term evolution-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system, considering relaying for cost-effective throughput enhancement and coverage extension. The LTE-A system includes all of the features of the LTE system and several new ones, the most important of which are: carrier aggregation, enhanced multi-antenna support, coordinated multiple point transmission and reception (CoMP), relaying and so on. The LTE system provides extensive support for deployment in spectrum allocations of various characteristics, with transmission bandwidths ranging from 1.4 MHz up to 20 MHz. In the LTE-A system, the transmission bandwidth can be further extended with carrier aggregation wherein multiple component carriers are aggregated and jointly used for transmission to/from a signal UE. In general, up to five component carriers can be aggregated, allowing for transmission bandwidth up to 100 MHz. In addition to wider bandwidth, LTE-A system is also expected to provide higher data rates and improved system performance. It will do this by further extending the support for multi-antenna transmission compared to the first release of LTE. For the downlink (DL), up to eight layers can be transmitted using an 8×8 antenna configuration. This allows for a peak spectral efficiency exceeding the requirement of 30 bits/s/Hz and implies a possibility for data rates beyond 1 Gbit/s in a 40 MHz bandwidth and even higher data rates with wider bandwidth.
LTE-A system also supports transmission and reception from multiple eNBs or transmission points and this technique is known as CoMP. CoMP can be used to enhance the transmission for cell edge UEs and also have better interference mitigation in system perspective.
For the correct operation of the wireless communication systems, only the data symbols transmitted in data channel is not enough. Downlink control information in downlink control channel (e.g. physical downlink control channel (PDCCH) in LTE and LTE-A systems) like DL grants which indicate the transmission formats (e.g. modulation format, channel coding rate, precoding matrix index, resource allocation, etc.) and UL grants which indicates the uplink parameters (e.g. modulation format, channel coding rate, precoding matrix index, resource allocation, etc.) are also necessary and should be transmitted in the control channel for a UE, in order to receive data channel and allocate uplink transmission in a right manner. For the cellular network, a base station needs to serve a lot of UEs. Therefore, the control channel transmitted by the base station contains multiple control information for different UEs. It is noted to make each mobile device know the position of its control information, lots of signaling may be needed. The control channel is usually blindly decoded and each UE has a UE-specific search space on the control channel to reduce the corresponding signaling.
A UE obtains control information dedicated to the UE through blind decoding PDCCHs, and all the PDCCHs the UE blindly decodes are PDCCH candidates in UE perspective. Nevertheless, during the blind decoding procedure, UEs suffer from decoding latency problem and may need to buffer a large number of symbols. Such additional complexity may generate a heavy burden for UEs. As a result, typically the number of total PDCCH candidates for each UE is a limited number. On the other hand, all PDCCH candidates for a certain UE in the UE-specific search space are contiguous among the control channel element (CCE) domain for simplicity.
As mentioned above, LTE-A system has several new features compared to conventional LTE system. The conventional PDCCH of LTE system works poorly with the addition of these new features. Therefore, enhanced PDCCH (EPDCCH) is introduced to have better cooperation with these new features.
An EPDCCH can be classified into two different types: localized transmission and distributed transmission. The design principle of distributed transmission is similar to legacy PDCCH. On the other hand, localized transmission aims at utilizing the MIMO beamforming gain and the frequency scheduling gain. The localized transmission of EPDCCH is placed localized on the time-frequency resources and is not interleaved. Yet, the channel frequency response may have poor gain among localized time-frequency resources. When constrained contiguous search space for EPDCCH is specified following a manner similar to that in conventional PDCCH, the base station may has a high probability that the base station can only schedule the EPDCCH among time-frequency resources with poor wireless channel gain and results in bad system performance.